add getCredentials method of AuthenticationToken as a remote source

Author: am0o0

java/ql/lib/ext/experimental/org.apache.shiro.authc.model.yml

@@ -0,0 +1,6 @@
+extensions:
+  - addsTo:
+      pack: codeql/java-all
+      extensible: sourceModel
+    data:
+      - ["org.apache.shiro.authc","AuthenticationToken",true,"getCredentials","()","","ReturnValue","remote","manual"]